My subject was a DAC with an unbalanced input (SPDIF). Do you think CMRR applies to unbalanced inputs? For a USB DAC, CMRR would only apply to the signal pair, but the grounds are where the problems are with digital systems.

I was talking about what I said - ability to reject CM noise is a desirable quality in a DAC. If you're not clear what CM noise is, Google can be your friend. Here's a link to get you started :

I'm fairly sure its down to the DAC, but there are qualities of DACs which don't get measured and specified in marketing materials. CM noise rejection is one such, but its nevertheless a crucial parameter. I've seen no DAC that has a measurement to date.

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